This invention relates to a process for polymerizing an aqueous solution of an .alpha.,.beta.-ethylenically unsaturated monomer and at least one crosslinking monomer to produce a water-swellable polymer.
Polymers which form hydrogels are used in fluid absorbing products such as disposable diapers, incontinent devices and sanitary napkins and are well known in the art. These polymers can be prepared by polymerization of .alpha.,.beta.-ethylenically unsaturated monomers with crosslinking agents. The development of improvements in the processes for preparing these water-swellable polymers is desirable to produce improved products, eliminate by-products and increase production capacity. The improvements in the processes however, should not sacrifice excellent free swell capacity and water-soluble polymer content properties of the polymer.
One technique to increase throughput of the polymer product is to increase the percent solids in the reaction mixture during solution polymerization of the monomer. In order to increase the percent solids, the amount of the solvent, typically water, present must be reduced during the polymerization or the monomer concentration must be increased prior to polymerization. However, a higher ratio of monomer to water also increases the apparent viscosity of the mixture resulting in increased stress on the agitator used during the polymerization to mix the solution and higher power requirements to achieve desired mixing. Such stress is undesirable due to increased wear on the equipment. Additionally, the apparent viscosity of the reaction mixture at higher solids levels is such that attaining that desired mixing with a conventional agitator is difficult, effectively preventing the desired increase in production capacity.
Presently, initiator components such as sodium sulfite are used which impart a polymerization rate more rapid than the mixing rate of the agitator. Therefore, it would be desirable to reduce the initial rate of polymerization. Accordingly, the present invention is such a method for slowing the initial rate of polymerization without sacrificing excellent free swell capacity and water-soluble polymer content properties of the polymer.